Thanks to a great advance in an electric power and electronic technology, the use of an inverter is significantly increasing in a motor/generator application field. As best seen in FIG. 1, a Voltage Source Inverter (VSI) is generally used in an electric and machine driving system. Various types of electric and machine systems may be applied to an inverter application; however an induction motor/generator or a synchronous machine is widely used for industrial applications because it has a higher quality. A rare earth-based permanent magnet synchronous machine is famous for an enhanced torque and output density and a high efficiency, but it has a disadvantage in the middle of operations in a flux weakening or field weakening section since it is hard to change in real time the magnetic flux of a rare earth-based permanent magnet. Despite the above mentioned disadvantage, the use of the permanent synchronous machines is growing in various application fields such as vehicles, wind power, etc. Since a limited number of countries is gifted to exclusively produce the rare earth materials which are used in the rare earth-based permanent magnets, it is hard to obtain enough rare earth materials due to the increasing use of such rare earth materials, which problems consequently cause an increase in costs.
Since an electric vehicle, a hybrid vehicle and a permanent magnet-based wind power generator continue to increase, it is expected that the rare earth-based permanent magnets necessarily adapted in the permanent magnet motor and the generator which are key elements in generating the driving forces for the system become harder and harder to obtain and use, so many researches for developing substitutable materials are in process. As a result of such researches, there are a method of reducing the use of the permanent magnets by changing the structures of the induction motor/generator, wound-rotor type synchronous machine, etc. and a method of substituting an expensive rare earth-based magnet with a cheaper permanent magnet like ferrite.
In case of the typical wound-rotor type synchronous machine, it needs to continuously supply magnetic field current to the wound rotor in order to form field magnetic fluxes. For this, electric power is connected to a field winding of the rotor. In order to externally supply electric current to the rotor of the synchronous machine, an exciter which requires the supply of external electric power is configured in such a way to attach a slip ring to the shaft of the rotor. When the typical exciter is used, the rotor requires separate electric power, and the control of the field fluxes requires a separate circuit for controlling the electric current of the rotor.
FIG. 2 is a view illustrating a half-wave rectified brushless synchronous machine from which a brush and an exciter are removed in an attempt to improve the problems of the exciter of the conventional wound-rotor type synchronous machine. The above mentioned synchronous machine has features in that similar with the typical wound-rotor type synchronous machine, a field winding is installed at the rotor. The field winding is not connected to an external exciter, but to a diode inside the rotor. Since there is not a separate exciter, the magnetic field current of the rotor can be controlled by an inverter connected to a stator. When a voltage with a frequency higher than a synchronous frequency is supplied to the winding of the stator in the circuit of the stator, the voltage and current of the high frequency components are supplied to the rotor with the aid of electromagnetic induction phenomenon. It is known that a current pulsation corresponding to an average value of a magnetic field current is supplied to the stator and the winding of the rotor in order to generate enough air gap flux density in the circuit formed of only the above mentioned diode. The magnetic field current may be supplied in the above mentioned way to the rotor, but a current pulsation components contained in the magnetic field current and the stator current may decrease the efficiency of all parts of the synchronous machine and may cause torque pulsation, which results in the decreased performance of the whole systems.